local s = "[5. Trade - City] [SomeCharacterName]: Testing Chat"
local result, _ = s:gsub("(%[%d+%. )Trade.-%]", "%1TR]")
print( result )

local text = "[5. Trade (Services) - City] [SomeCharacterName]: Testing Chat"
local res, _ = text:gsub("(%[%d+%. )Trade %([^()]*%).-%]", "%1TRS]")
print( res )